Laser diode read driver

ABSTRACT

A laser diode read driver is described. A first transistor operative for producing a first voltage in response to receiving a first current signal. A first resistor is coupled between the first transistor and a low voltage supply. A first transconductor having a first input is coupled to receive the first voltage, wherein the transconductor produces a second current signal in response to differences between signals received on the first input and a second input. A second transistor is coupled to the second input, and operative for producing a third current signal in response to receiving the second current signal. A third transistor is coupled to the second transistor and the second input, the third transistor operative for producing an output current signal in response to receiving the third current signal, wherein the first transistor is scaled to the first transistor by the inverse of a gain factor. A second resistor is coupled between the third transistor and a low voltage supply, wherein the first resistor is scaled to the second resistor by the gain factor.

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to jointly owned U.S.Provisional Application corresponding to application No. 61/186,299entitled “Laser Diode Read Driver.” This provisional application wasfiled on Jun. 11, 2009.

DESCRIPTION OF RELATED ART

With the evolution of electronic devices, there is a continual demandfor enhanced speed, capacity and efficiency in various areas includingelectronic data storage. Motivators for this evolution may be theincreasing interest in video (e.g., movies, family videos), audio (e.g.,songs, books), and images (e.g., pictures). Optical disk drives haveemerged as one viable solution for supplying removable high capacitystorage. When these drives include light sources, signals sent to thesesources should be properly processed so these sources emit theappropriate light for reading and writing data optically.

BRIEF DESCRIPTION OF THE DRAWINGS

The laser diode read driver within the laser diode driver signalprocessing system may be better understood with reference to thefollowing figures. The components within the figures are not necessarilyto scale, emphasis instead being placed upon clearly illustrating theprinciples of the invention. Moreover, in the figures, like referencenumerals designate corresponding parts or blocks throughout thedifferent views.

FIG. 1A, is a system drawing illustrating components within an opticaldisk drive

FIG. 1B is an enlarged view of the innovative laser driver, which may bea laser diode drive (LDD).

FIG. 2 is a simplified circuit diagram for a first implementation of theLDRD that sinks current.

FIG. 3 is a simplified circuit diagram for a second implementation ofthe LDRD that sources current.

FIG. 4 is one implementation of a detailed circuit diagram of the LDRDthat sinks current.

FIG. 5 is an actual circuit diagram of a laser read driver that sourcescurrent.

While the laser diode read driver is susceptible to variousmodifications and alternative forms, specific embodiments have beenshown by way of example in the drawings and subsequently are describedin detail. It should be understood, however, that the description hereinof specific embodiments is not intended to limit the motion conversionsystem to the particular forms disclosed. In contrast, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the motion conversion as defined by thisdocument.

DETAILED DESCRIPTION OF EMBODIMENTS

As used in the specification and the appended claim(s), the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Similarly, “optional” or “optionally” meansthat the subsequently described event or circumstance may or may notoccur, and that the description includes instances where the event orcircumstance occurs and instances where it does not.

Turning now to FIG. 1A, is a system drawing illustrating componentswithin an optical disk drive 100. A controller 102 monitors the outputlight power level of a laser diode 115 using a Monitor PD 104, ormonitor photodiode, and an RF, or radio frequency, preamplifier 106.This controller can keep an expected power level by changing an inputcontrol current of a laser driver 110 through an APC, or auto powercontrolling, feedback loop, even if a light source 115 such as a laserdiode, has many changes of the output power due to various conditionchanges, such as temperature etc.

Also, the controller 102 sets the enable signal for switching somecurrent channels of the laser driver 110, which arranges a data writingpulse. In the case of data reading, the controller 102 may only set theDC current by disabling the switching and applying the designatedcurrent. In the case of data writing, the controller 102 applies someadjustment signals, or enable-switching signals, to arrange the writingpulse waveform as a combination of switched current pulses. The powerlevel can be changed as each switching channel has its own designatedcurrent. The controller 102 can arrange these designated currents basedon the Monitor PD 101 output with some detecting function in the RFpreamplifier 106. At the very least, this controller has two powercontrol levels, one for the read power and one for the write power.Sometimes the controller may get the top, bottom, or average level of awriting pulse and perform calculations to control some power levelsindependently.

As illustrated in this figure, the laser driver 110 sends a signal thatprompts an associated light source 115 (e.g., laser diode) to emitlight. The light source 115 may emit light at any of a number ofwavelengths (e.g., 400 nm, 650 nm, 780 nm). Light from this sourcecontacts an associated optical media 120, such as a compact disc (CD),blue ray device (Blu-ray), or digital versatile disk (DVD). Lightcontacting the optical media can either facilitate data storage or dataretrieval from the optical media 117.

FIG. 1B is an enlarged view of the innovative laser driver 110, whichmay be a laser diode driver (LDD). The LDD 110 is an integrated, fullyprogrammable, multi-function product that controls and drives laserswithin optical drives as described with reference to FIG. 1A. Morespecifically, the LDD 110 can facilitate reading, writing, and erasinghigh capacity disks (e.g., capacities greater than approximately 50Gbytes/disk). The LDD 110 also has low noise (e.g., noise ofapproximately 0.5 nA/Hẑ2), high speed (e.g., 800 Mb/s) and high current(e.g., approximately 1 amp). Any numbers included in this applicationare for illustrative purposes only and numerous alternativeimplementations may result from selecting different quantitative values.

At a high level, the LDD 110 may include a current generator 150.Generally, the current generator 150 receives some input signalsassociated with several input channels 153, which have an associatedinput current. This current generator 150 works in tandem with a currentdriver 160 and scales the input current by some gain factor. As aresult, the current generator 150 and current driver 160 can control theamount of current for each output channel 195. For the input signalsthat the current generator 150 receives, it transmits output signalsthat a current switch 155 receives. The current switch 155 decides whichof the input channels should be turned on or turned off. For thechannels that should be turned on, the current switch 155 makes thosechannels active. Similarly, the current switch 155 inactivates thechannels that should be turned off and transmits output signalsreflecting this change. The current driver 160 receives these outputsignals from the current switch 155 as input signals. This currentdriver is the last current gain stage and drives the laser diodedirectly. In other words, the output signals from the current driver 160also serve as output signals for the LDD 110, which are used in drivingthe lasers, or light source 115.

In addition to the above-mentioned devices, the LDD 110 includesadditional components. A serial interface (I/F) 170 has several inputs(e.g., serial data enable, serial data, serial clock) that may be usedfor programming the gain, enabling channels, and turning on the LDD. Thetiming generator 175 receives various channel enable inputs 190. Thoughthere arc five channel enable inputs that are shown in FIG. 1B, the LDD110 may have any number of channel enable inputs, such as two, six, orthe like. The timing generator 175 determines the time at which a givenoutput channel will be either turned on or turned off. The LDD 110 alsoincludes a high frequency modulator (HFM) 180 and voltage/temperaturemonitor (V/Temp Monitor) 185. The HFM 180 modulates the output currentfor mode-hopping noise reduction of the laser diodes. Thevoltage/temperature monitor 185 monitors the laser diode voltage dropand on-chip temperature. One skilled in the art will appreciate thatnumerous alternative implementations may result from either adding orremoving any or several of the blocks within the LDD 110.

Though not illustrated, an integrated circuit for the LDD 110 generallyhas four switching, or write channels and one static, or read channel.The read channel should accommodate a very large dynamic range fromseveral milliamps to hundreds of milliamps with good accuracy. Anotherdesign constraint is that the associated integrated circuit should havevery low noise and be essentially immune to coupling from the switchingchannels. To meet these constraints, the LDD 110 includes a Laser DiodeRead Driver (LDRD) 165. The LDRD 165 has a large dynamic range, lownoise, good accuracy, and is essentially immune to the switchingchannels coupling; in other words, this LDRD does not ring. The outputof the LDRD 165 can become the output of the LDD 110. The input coniesfrom the current generator block 150.

FIG. 2 is a simplified circuit diagram 200 for a first implementation ofthe LDRD 165 that sinks current. For this implementation, the laserdiode's cathode connects to the pin I_(OUT) and the laser diode's anodeconnects to a positive voltage supply. When there is a desired outputcurrent, the LDRD 165 can be designed to produce this desired current asillustrated in the circuit diagram 200. For example, the desired outputcurrent may be I_(OUT) and the circuit diagram 200 may have a gain Kassociated with it. To produce this output current, the circuit diagram200 receives an input reference current I_(OUT)/K shown as currentsource 205, where K is the gain factor; this input current comes from aprevious stage in the current generator block 150. As this referencecurrent enters this circuit, the current reaches ground by travelingthrough transistor 210 and resistor 215. While the transistor 210 isshown as an npn bipolar junction transistor, other implementations mayresult from using different transistor types. The transistor 210 is alsoa diode-connected transistor. The size of transistor 210 can be scaledto an output transistor 220 by the inverse of the gain factor, or 1/K,(e.g., area of transistor 210 may equal area of transistor 220*1/K). Ascurrent flows from current source 203 through transistor 210, it reachesresistor 215 and then encounters ground. As transistor 210 is scaled totransistor 220, resistor 215 can be scaled to the output resistor 225;for example, the value of resistor 215 can be the product of resistor225 and the gain K. Matching the device 210 with the device 220 and thedevice 215 with the device 225 can improve the accuracy of the outputcurrent in relation to the input current I_(OUT)/K.

The input reference current I_(OUT)/K 205 sets a reference voltage atthe V_(N) terminal 232 of the transconductor 230. The transconductor 230has two input terminals and produces a current signal reflective ofdifferences between signals received on its input terminals. Asmentioned above, the transconductor 230 includes a V_(N) terminal 232and V_(P) terminal 234 where V_(N) is the voltage applied to theterminal 232 and V_(P) is a voltage applied to the terminal 234. Thevalues for these voltages may be the sum of (I_(OUT)/K)*Resistor 215 andthe voltage of diode connected transistor 210, or the like. Thetransconductor 230 produces an output current signal on terminal 236that reflects a difference of the signals received on the terminal 232and the terminal 234. The output current signal has an associated outputcurrent I where I=GM*(VP−VN). In this formula, GM is thetransconductance of the transconductor 230, which may have a value of 20uS or the like.

the output current signal emerges from the transconductor 230, it drivesthe capacitor 240. The size of this capacitor for this particularapplication is around 15 pF The capacitor 240 can filter noise presentin the output current signal that may be associated with a previousstage in the laser diode driver 110. In other words, noise. in theoutput signals from the current generator 150 (see FIG. 1B) may appearas noise on the input terminal 232, which would appear as noise in theoutput current signal on the terminal 236. It is this noise in theoutput current signal that capacitor 240 can filter. The size for thiscapacitor may be selected based on design parameters to get a desiredamount of filtering.

The output current signal from the transconductor 230 also drives ametal oxide semiconductor (MOS) transistor 250. While shown as a MOStransistor, one skilled in the art will appreciate that the specifictype of transistors within the LDRD 165 and the circuit 200 may varydepending on design objectives. This output current signal drives thegate of the transistor 250 to a voltage such that the voltage V_(P)equals the voltage V_(N) by outputting a current into the transistor 260and the resistor 265, which goes to a low voltage supply, which istypically ground. The size of the transistor 260 can scale to thetransistor 210 or the transistor 220, if desired. Similarly, theresistance of the resistor 265 can scale to the resistor 215 or theresistor 225, if desired. In addition, the transistor 260 and theresistor 265 form a current mirror 270 that connects to the base ofoutput transistor 220, the terminal 234 of the transconductor 230, thedrain of the transistor 250, and the low voltage supply or ground.

The LDRD 165 illustrated by the circuit diagram 200 has an effectiveoperation. As briefly mentioned above, this circuit diagram includes ahigh voltage supply V_(cc), which may have a voltage of approximately 5Vassociated with it. Current Source 205, capacitor 240, and transistor250 all connect to this voltage supply. In contrast, resistors 215, 225,and 265 all connect to the low voltage supply, or ground. Due to theclosed loop or the connection of the current mirror leg 270, thetransconductor 230, and transistor 250, the voltage at the base of thetransistor 260 and the base of the transistor 220 will be the same asthe voltage on the base of the transistor 210. In other words, thevoltage Vn at the base of transistor 210 terminal 232 equals the voltageVp on terminal 234 as explained above, which is applied to the bases ofthe transistor 220 and transistor 260. Because transistor 220 andresistor 225 are scaled to the transistor 210 and the resistor 215, theoutput current I_(out) or current emerging from the LDRD 165 and thecircuit diagram 200 will be a scaled replica of the input current by thegain factor K.

High frequency coupling from the collector-base junction of Q2 is alsofiltered by the loop. As coupling current is injected into node 231, thetransconductor 230 will respond by outputting a current, but it willonly respond as fast as the loop frequency response, which is dictatedby several parameters, specifically, the value of transconductance andthe value of capacitance for capacitor 240.

The simplified circuit diagram 200 is merely one of many possibleimplementations of the LDRD 165. In fact, numerous alternativeimplementations can result, without departing from the inventive aspectdescribed in this document. For example, an alternative implementationcan result from removing the current mirror leg 270. Another alternativeimplementation can result from replacing the current mirror 270 with apassive device (e.g., a resistor). FIG. 3 is a simplified circuitdiagram 300 for a second implementation of the LDRD 165 that sourcescurrent. In this implementation, the laser diode cathode is connected toground, while the laser diode's anode is connected to the pin I_(OUT).One skilled in the art will appreciate that the circuit diagram 300 isessentially “flipped” relative to the circuit diagram 200. Because thedescription of the operation of circuit diagram 200 is applicable to thecircuit diagram 300, the operation of the circuit diagram 300 is notseparately described.

FIG. 4 is one implementation of a detailed circuit diagram 400 of theLDRD 165 that sinks current; this operates generally as described withreference to FIG. 2. This circuit diagram includes a current mirror thatprovides bias current for the cell's operation; a current mirror thatattenuates an input bias current to be used for the transconductor 230.As illustrated, the transconductor 230 may be composed of fourtransistors 332-338. The current mirror 270 is also shown as atransistor and resistor, but other alternatives are possible. Thecircuit diagram 100 also includes transistor 440 and transistor 445 thatarc connected to the input terminals of the transconductor 230. Thesetransistors act as emitter followers into the inputs of thetransconductor 230. In other words, they shift the voltage up by a diodeso the circuit has enough headroom for proper operation. A transistor450 sets a tail current for the transconductor 230, which determines thefrequency response of the current mirror. Like FIG. 3, FIG. 5 is asecond implementation of a circuit diagram 500 that sources current withsome similar components to the components described with reference toFIG. 4. In this implementation, the transconductor 230 includestransistors 532-538; transistors 540, 545 act as emitter followers intothe inputs of the transconductor 230. The transistor 550 sets the tailcurrent for this transconductor.

The LDRD 165 provides a very accurate representation of the inputcurrent as scaled by a gain factor at the output for a very largedynamic range. This innovative LDRD can use very little power dependingon the selected gain, device sizes, and resistor sizes. In addition,noise from prior stages can be easily filtered without ringing due tohigh frequency coupling. Unlike conventional solutions, the LDRD 165does not sacrifice accuracy or require a beta-helper.

While various embodiments of the laser diode read driver have beendescribed, it may be apparent to those of ordinary skill in the art thatmany more embodiments and implementations are possible that are withinthe scope of this system. Although certain aspects of the laser dioderead driver may be described in relation to specific techniques orstructures, the teachings and principles of the present system are notlimited solely to such examples. All such modifications are intended tobe included within the scope of this disclosure and the present laserdiode read driver and protected by the following claim(s).

1. A laser diode read driver, comprising: a first transistor operativefor producing a first voltage in response to receiving a first currentsignal; a first resistor coupled between the first transistor and a lowvoltage supply; a first transconductor having a first input coupled toreceive the first voltage, wherein the transconductor produces a secondcurrent signal in response to differences between signals received onthe first input and a second input; a second transistor coupled to thesecond input, and operative for producing a third current signal inresponse to receiving the second current signal; a third transistorcoupled to the second transistor and the second input, the thirdtransistor operative for producing an output current signal in responseto receiving the third current signal, wherein the first transistor isscaled to the first transistor by the inverse of a gain factor; and asecond resistor coupled between the third transistor and a low voltagesupply, wherein the first resistor is scaled to the second resistor bythe gain factor.
 2. The laser diode read driver of claim 1, furthercomprising a third resistor coupled to the second input, the secondtransistor, the third transistor, and the low voltage supply.
 3. Thelaser diode read driver of claim 1, further comprising a capacitorcoupled to the output of the transconductor, the second transistor, anda high voltage supply, wherein the capacitor is operative for filteringnoise from the output current signal.
 4. The laser diode read driver ofclaim 1, wherein coupling the third transistor to the second input andthe second transistor filters high frequency coupling associated with anoutput transistor.
 5. The laser diode driver of claim 1, furthercomprising first and second emitter followers respectively coupled tothe first input and the second input.
 6. The laser diode driver of claim7 further comprising a fourth transistor coupled to either the firstinput or the second input and operative for setting a tail currentassociated with the transconductor.
 7. A laser diode read driver,comprising: a first transistor operative for producing a first voltagein response to receiving a first current signal; a first resistorcoupled between the first transistor and a low voltage supply; a firsttransconductor having a first input coupled to receive the firstvoltage, wherein the transconductor produces a second current signal inresponse to differences between signals received on the first input anda second input; a second transistor coupled to the second input andcoupled to produce a third current signal in response to receiving thesecond current signal; a third transistor coupled to the secondtransistor and the second input, the third transistor operative forproducing an output current signal in response to receiving a thirdcurrent signal, wherein the first transistor is scaled to the firsttransistor by the inverse of a gain factor; a second resistor coupledbetween the third transistor and a low voltage supply, wherein the firstresistor is scaled to the second resistor by the gain factor; and acurrent mirror coupled to the second input, the second transistor, andthe third transistor, wherein the current mirror produces a secondvoltage on the second input, and the output current signal that is ascaled version of the first current signal.
 8. The laser diode readdriver of claim 7, further comprising a capacitor coupled to the outputof the transconductor, the second transistor, and a high voltage supply,wherein the capacitor is operative for filtering noise from the outputcurrent signal.
 9. The laser diode read driver of claim 7, whereincoupling the third transistor to the second input and the secondtransistor filters high frequency coupling associated with an outputtransistor.
 10. The laser diode driver of claim 7, further comprisingfirst and second emitter followers respectively coupled to the firstinput and the second input.
 11. The laser diode driver of claim 10further comprising a fourth transistor coupled to either the first inputor the second input and operative for setting a tail current associatedwith the transconductor.
 12. An optical disk drive system comprising: acurrent generator for receiving input signals; a current switch coupledto receive output signals from the current generator; a current drivercoupled to receive output signals from the current switch, the currenthaving a laser diode read driver, the laser diode read drivercomprising: a first transistor coupled to a high voltage source andoperative for producing a first voltage in response to receiving a firstcurrent signal; a first transconductor having a first input coupled toreceive the first voltage, wherein the transconductor produces a secondcurrent signal in response to differences between signals received onthe first input and a second input; and a second transistor coupled tothe second input and coupled to produce a third current signal inresponse to receiving the second current signal; a current mirrorcoupled to the second input and the second transistor, wherein thecurrent mirror produces a second voltage on the second input and anoutput current signal that is a scaled version of the first current 13.The optical disk drive of claim 12, wherein the laser diode react driverfurther comprises a capacitor coupled to the output of thetransconductor, the second transistor, and a high voltage supply,wherein the capacitor is operative for filtering noise from the outputcurrent signal.
 14. The optical disk drive of claim 12, wherein thelaser diode read driver further comprises: a third transistor coupled tothe second transistor and the second input, the third transistoroperative for producing an output current signal in response toreceiving a third current signal, wherein the first transistor is scaledto the first transistor by the inverse of a gain factor; and a secondresistor coupled between the third transistor and a low voltage supply,wherein the first resistor is scaled to the second resistor by the gainfactor.
 15. The optical disk drive of claim 14, wherein coupling thethird transistor to the second input and the second transistor filtershigh frequency coupling associated with an output transistor.
 16. Theoptical disk drive of claim 12, further comprising first and secondemitter followers respectively coupled to the first input and the secondinput.
 17. The optical disk drive of claim 12, further comprising afourth transistor coupled to either the first input or the second inputand operative for setting a tail current associated with thetransconductor.